Chronic lymphocytic leukemia (CLL) is the most prevalent leukemia in the western world and is considered incurable. Work on the biology of CLL has revealed that accessory cells in the leukemia microenvironment can promote leukemia-cell survival and resistance to chemotherapy in vitro, and presumably in vivo. We have characterized a type of accessory cell, called nurselike cells (NLC), which can protect leukemia cells from apoptosis, and defined several ligand-receptor interactions that together can account for such activity. We also have observed that CLL cells from different patients can vary in their relative dependency on such accessory cells for survival in vitro, and presumably in vivo. Lenalidomide is not directly cytotoxic to CLL cells, but has apparent clinical activity in CLL. The mechanism(s) for tumor-cell reduction in patients treated with this drug is unknown. We found that lenalidomide can mitigate the protective influence of NLC on CLL cell survival in vitro. We hypothesize that this drug functions at least in part by modulating the capacity of CLL cells to respond to the survival signals received from NLC in the leukemia-cell microenvironment. Furthermore, we hypothesize that patients with leukemia cells with high dependency on NLC for survival might enjoy greater clinical responses to lenalidomide than patients with CLL cells that have low NLC-dependency. In addition, we have identified immunohistologic means with which to measure the relative abundance of NLC in the marrow of patients with CLL. We speculate that patients with high NLC content in the marrow might have different responses to chemotherapeutic agents and/or to lenalidomide than patients with low NLC content. We have the outstanding opportunity to test these hypotheses in the context of a national clinical trial that we organized, evaluating the activity of lenalidomide, and then lenalidomide and rituximab, in previously untreated CLL patients who require therapy by current treatment guidelines. For this we have the following specific aims: Specific aims: 1. Determine whether high NLC dependency is associated with response to lenalidomide. 2. Assess whether treatment with lenalidomide impairs the capacity of CLL cells to receive survival signals from NLC in vitro. 3. Assess the clinical significance of the relative NLC content in pretreatment marrow biopsy specimens with regard to the response to lenalidomide or to standard chemotherapy. Through work performed on this proposal we will determine whether clinical activity of lenalidomide is related to its capacity to affect the leukemia microenvironment and gain insight into patient-features that have a potential bearing on the response to therapy and overall survival. Public Health Relevance: The proposal is a bidirectional translational research effort designed to understand the impact of lenalidomide on the chronic lymphocytic leukemia (CLL) microenvironment that accompanies a national clinical study investigating this agent for the frontline treatment of CLL. Two potential novel biomarkers representing the CLL microenvironment will be evaluated, one prospectively and one retrospectively for utility in predicting clinical response to this agent. Investigation of the mechanisms responsible for the clinical activity of lenalidomide in CLL will be performed by ex-vivo evaluation of the subject's leukemia cells during therapy.